Various diene-based elastomers may be prepared, for example, by blending the elastomer(s) with rubber reinforcing filler such as rubber reinforcing carbon black and silica reinforcement, particularly precipitated silica reinforcement, together with a silica coupling agent comprised of a bis(3-triethoxysilylpropyl)polysulfide or an organoalkoxymercaptosilane to aid in coupling the silica to the elastomer and enhancing its rubber reinforcing effect. Preparation of such rubber compositions are well known to those having skill in such art.
In an alternative embodiment, the precipitated silica reinforcement may be provided as a pre-hydrophobated precipitated silica which has been hydrophobated by reaction with the silica coupling agent prior to its addition to the rubber composition. The precipitated silica itself is somewhat hydrophilic which therefore tends to attracted more to itself (resulting in the silica particles tending to agglomerate together) rather than to disperse within elastomers in which it is desired to provide reinforcement.
Pre-treatment by pre-hydrophobating the precipitated silica by reaction of hydroxyl groups (e.g. silanol groups) on the precipitated silica with at least one of organoalkoxymercaptosilane and bis(3-alkoxysilylalkyl)polysulfide tends to hydrophobate the precipitated silica to provide a polarity more readily dispersible within the diene rubber containing rubber composition. Further, addition of such organoalkoxymercaptosilane and bis(3-alkoxysilylalkyl)polysulfide silica coupling agents to the rubber composition to promote coupling of the precipitated silica to diene-based elastomers in the rubber composition might not be needed unless otherwise desired.
However, although the mechanism may not be completely understood, relatively high loadings of the rubber composition with such pre-hydrophobated (pre-silica coupler treated) precipitated silica can significantly increase the sulfur cure-rate of the rubber composition to an extent of promoting disadvantageously pre-curing of the rubber composition which is sometimes referred to as scorching. Perhaps the increased cure rate might be associated with, or at least partially a result of, a reduced salinization occurring due to absence of ethanol byproduct in the pre-hydrophobated precipitated silica used for the experiments.
Therefore it is proposed to evaluate counterbalancing such apparently induced increased rubber cure rate by addition of a processing aid comprised of a pre-formed zinc fatty acid soap with such pre-hydrophobated precipitated silica to the rubber composition to therefore reduce the fast cure promotion by use of the pre-hydrophobated silica.
It is recognized that the rubber composition is normally compounded with a combination of zinc oxide and fatty acids (e.g. comprised of a combination of stearic, palmitic and oleic acids) which form a zinc salt of the fatty acids in situ within the rubber mixture as it is being mixed.
However, for this invention, is proposed to evaluate the addition of a pre-formed zinc salt of fatty acid(s) with the pre-hydrophobated precipitated silica to the rubber composition without adding, or minimalizing addition of, zinc oxide to the rubber composition and thereby waiting for an in situ formation of zinc salt of fatty acids during a mixing of a small amount of zinc oxide and fatty acids with the rubber composition
A benefit of inclusion of precipitated silica reinforcing filler in a tire tread rubber composition, including inclusion of precipitated silica having been pre-hydrophobated by pre-reaction with the silica coupler in a tire tread rubber composition, is to normally promote a beneficially lower rolling resistance for a tire tread comprised of the cured rubber composition and thereby a beneficially increased fuel economy for an associated vehicle. An indication of predictive beneficial reduction in tire tread rolling resistance is an increase in rebound property of the rubber composition as well as a decrease in its tangent delta at low strain (low dynamic elongation) which are properties well known to those having skill in such art.
However, it is anticipated by such addition of pre-formed zinc fatty acid salt to the pre-hydrophobated silica-containing rubber composition can promote a disadvantageously reduced increase in the rubber composition's rebound property and a disadvantageously less reduction in its tangent delta at low strain property and therefore a disadvantageously less predictive reduction in rolling resistance for a tire tread of such rubber composition.
Therefore, it is further desired to provide a rubber composition to accommodate such combination of pre-hydrophobated silica and pre-formed zinc salt of fatty acid(s) addition to the rubber composition.
For such rubber composition, it is proposed to provide two functionalized styrene/butadiene elastomers in the rubber composition with functional groups reactive with hydroxyl groups contained on the pre-hydrophobated precipitated silica. Representative of such functional styrene/butadiene elastomers is a first styrene/butadiene elastomer containing a combination of siloxy and amine groups and a second styrene/butadiene elastomer containing a combination of siloxy and thiol groups.
In the description of this invention, the terms “rubber”, “elastomer” and “rubbery polymer” may be used interchangeably unless otherwise indicated. The terms “cured” and “vulcanized” may be used interchangeably unless otherwise indicated.
The term “phr” refers to parts by weight of an ingredient per 100 parts by weight of rubber in a rubber composition.
Such terms are known to those having skill in such art.